Verify checksums when downloading assets

In the last few weeks (I think) GH added checksums to the API reponses
for release assets. Use them to verify the downloaded files.

Also bump the chunk size a bit while at it, it was quite small..

msys2_autobuild/gh.py

@@ -4,10 +4,10 @@ import shutil
 import sys
 import tempfile
 import time
+import hashlib
 from contextlib import contextmanager
 from datetime import datetime, timezone
 from functools import lru_cache
-from hashlib import sha256
 from pathlib import Path
 from typing import Any, Dict, Generator, List, Optional, Callable
 
@@ -153,8 +153,10 @@ def download_asset(asset: GitReleaseAsset, target_path: str,
         fd, temppath = tempfile.mkstemp()
         try:
             os.chmod(temppath, 0o644)
+            with verify_asset_digest(asset) as hash:
                 with os.fdopen(fd, "wb") as h:
-                for chunk in r.iter_content(4096):
+                    for chunk in r.iter_content(256 * 1024):
+                        hash.update(chunk)
                         h.write(chunk)
             mtime_ns = get_asset_mtime_ns(asset)
             os.utime(temppath, ns=(mtime_ns, mtime_ns))
@@ -171,7 +173,7 @@ def download_asset(asset: GitReleaseAsset, target_path: str,
 def get_gh_asset_name(basename: PathLike, text: bool = False) -> str:
     # GitHub will throw out charaters like '~' or '='. It also doesn't like
     # when there is no file extension and will try to add one
-    return sha256(str(basename).encode("utf-8")).hexdigest() + (".bin" if not text else ".txt")
+    return hashlib.sha256(str(basename).encode("utf-8")).hexdigest() + (".bin" if not text else ".txt")
 
 
 def get_asset_filename(asset: GitReleaseAsset) -> str:
@@ -183,6 +185,28 @@ def get_asset_filename(asset: GitReleaseAsset) -> str:
         return asset.label
 
 
+@contextmanager
+def verify_asset_digest(asset: GitReleaseAsset) -> Generator[Any]:
+    type_, value = get_asset_digest(asset)
+    value = value.lower()
+    h = hashlib.new(type_)
+    try:
+        yield h
+    finally:
+        hexdigest = h.hexdigest().lower()
+        if h.hexdigest() != value:
+            raise Exception(f"Digest mismatch for asset {get_asset_filename(asset)}: "
+                            f"got {hexdigest}, expected {value}")
+
+
+def get_asset_digest(asset: GitReleaseAsset) -> tuple[str, str]:
+    # https://github.com/PyGithub/PyGithub/issues/3324
+    digest = asset._rawData['digest']
+    assert digest
+    type_, value = asset._rawData['digest'].split(":", 1)
+    return type_, value
+
+
 def is_asset_from_gha(asset: GitReleaseAsset) -> bool:
     """If the asset was uploaded from CI via GHA"""